In the glass and window industry, customers who are contemplating the purchase of high quality and high performance windows want to be assured that the windows will meet their needs. For example, a single pane window transmits about 90% of the sun's energy, while a double pane window transmits about 45%. Also, a single pane window reduces UV energy by about 22-28% and a double pane window about 40%, while a high performance window can exceed an 80% reduction.
However, the window's performance characteristics, such as the amount of solar and UV transmission reduction, are invisible to the customer. In order to demonstrate the window's effectiveness, a sales associate will often use a portable light sensing meter and a portable light source to demonstrate the window's features.
Often, the light source used on the demonstration is a single light source (for example, a heat lamp or a UV “black” light) to mimic the light energy provided by the sun. However, these light sources have limited light spectrum that is not as broad as the natural sunlight spectrum. That is, the light source from such products does not provide energy across the entire light spectrum.
Also, the portable light meters have a limited range of sensitivity. That is, the light sensor in the meter does not have a “flat” or “the same” response for every wavelength across the entire light spectrum. When the light sources are used with the portable light meters, the meters do not always accurately produce a transmission measurement that is the same as the window manufacturer's “laboratory-tested” specifications.
The manufacturer's laboratory-tested specifications are typically the result of highly sophisticated testing using expensive and highly accurate equipment. In the industry, many laboratory measurements are performed using a spectrometer that has a light source over the whole spectrum and a prism to break down the spectrum to provide a transmission percentage to each wavelength. The percentages then can be averaged together to give a total transmission for a given spectrum (for example, solar, UV, visible). Mathematical calculations are performed which allow the user to select certain light frequency ranges. Then, individual wavelength measurements can be made on each uniquely defined light sensing pixel or element, and the resulting summation of the data can be made in the laboratory. Unfortunately, this type of equipment is either unavailable or impractical to use in the commercial or sales environment.
Therefore, the results from a limited range light source and a limited range sensor are used as the “true” or “field-tested” transmission performance of a broader light spectrum. There is generally no correlation, however, between the “laboratory-tested” measurements and the “field-tested” measurements. This poses a problem since the customer is expecting to see the same “laboratory-tested” measurements that are set forth in the window manufacturer's specifications.
In one example, the common practice among window industries' sale presentations is to use a solar meter in conjunction with a heat lamp to demonstrate the percentage of solar energy blocked by their product. The sensor in the meter has limited range, as shown in FIG. 1, and has a highly nonlinear output over the light wavelength range that the sensor can measure. The common heat lamp has a defined light frequency range, as shown in FIG. 2.
Glass manufacturers generally list the percentage of solar transmission based on the entire solar energy range. The solar spectrum is shown in FIG. 3. However, due to the above-described limitations of the heat lamp and the limitations of sensor technology available, the existing meter's transmission measurements do not equal the manufacturer's data sheets.
Thus, there exists a need for an efficient system for correlating field-tested measurement with the manufacturer's stated laboratory-tested measurements.